How do I make the most minimal vanilla theme possible with nothing but raw content?

The most minimal theme possible has two files: style.css and index.php.

style.css

/* Theme Name: Bare-bones theme*/

index.php

<?php
if( have_posts() ) {
    while( have_posts() ) {
        the_post();
        the_content();
    }
}

I’m not sure you could make it more “vanilla” than that. Keep in mind that any HTML markup within the post’s content will still be returned.

See Theme Development for more information, or the WP REST API plugin for an alternative approach to returning data without additional markup.